1) Technical Background
The present invention relates to overburden drilling equipment, wherein the overburden drilling equipment comprises a drill bit body having a connection section at a rear end for connection to a percussive unit such as a down-the-hole hammer and defining a rotational axis of a drill bit, and a casing shoe for attachment to a casing, the casing shoe connected to the drill bit body by mutual engagement of a groove and a collar in a lap-joint.
2) Prior Art
Overburden drilling equipment is previously disclosed in Ilomaki U.S. Pat. No. 5,255,960, for example. It has been found favorable to construct overburden drilling Equipment such that the casing and the rock drill bit are relatively rotatable while simultaneously axially connected, so that the drill bit pulls the casing along as the drill bit advances in the ground. Such relative rotation is achieved in the above-named patent by a lap-joint wherein the casing, or a casing shoe attached to the front of casing, radially and axially overlaps the drill bit and is pulled along therewith. It has now been found that removal of the drilling equipment from a bore is complicated by problems such as tendencies for the equipment to stick in the bore, wherein damage can occur to the casing shoe.
An object of the present invention is to provide an overburden drilling equipment, which obviates the above-mentioned drawbacks.